cmd/compile: don't clear ptrmask in fillptrmask

cmd/compile: don't clear ptrmask in fillptrmask

It is only ever called with a newly allocated slice.

This clearing code dates back to the C version of the compiler,
in which the function started like this:

static void
gengcmask(Type *t, uint8 gcmask[16])
{
	...

	memset(gcmask, 0, 16);
	if(!haspointers(t))
		return;

That memset was required for C, but not for Go.

Change-Id: I6fceb99b2dc8682685dca2e4289fcd58e2e5a0e0
Reviewed-on: https://go-review.googlesource.com/c/go/+/718340
LUCI-TryBot-Result: Go LUCI <golang-scoped@luci-project-accounts.iam.gserviceaccount.com>
Reviewed-by: Keith Randall <khr@golang.org>
Auto-Submit: Ian Lance Taylor <iant@golang.org>
Reviewed-by: Keith Randall <khr@google.com>
Reviewed-by: Junyang Shao <shaojunyang@google.com>